PCEXPMAP (Jan96) ftools.rosat PCEXPMAP (Jan96)
NAME
pcexpmap -- creates an exposure map for a given ROSAT PSPC dataset
with an option to use devignetted detector map
USAGE
pcexpmap evrfil attfil gtifil yes outfil
or
pcexpmap evrfil attfil gtifil no dmapfil outfil
DESCRIPTION
This task creates the band-correct exposure map for a ROSAT PSPC
pointed observation. (The task is essentially an FTOOLized version
of Steve Snowden's CAST_EXP code). Detector maps created from the
ROSAT All-Sky Survey data are required. The output is a 512x512
FITS image of the whole PSPC field of view (with pixels 14.94733
arcsec per side; representing a blocking factor of 16 over the raw
[0.9341875 arcsec] pixelsize) of the effective exposure time (in
seconds) at that position. The effects of vignetting (for a
spectrum equal to the mean spectrum of the X-ray background in the
PSPC band) and spatial variations in the efficiency of the detector
are included (via the detector maps), along with detector deadtime
effects (this code).
The program follows the suggestions of Snowden et al. (1992, ApJ,
393 819) and Plucinsky et al. (1993, ApJ, 418, 519) to exclude
regions near the edges of the PSPC which are strongly affected by
the particle background, the "bright line" regions. These regions
are set to have zero exposure time. The program also assumes that a
selection has been done on the data to exclude all events which
follow within 0.35 ms of a "precursor" event. This excludes some of
the low pulse-height "after-Pulse" signal which affects data
collected after 1992 May.
In brief, the attitude and GTI files are used to construct a
matrix of the time the instrument spent at each pointing position
(X,Y relative to the nominal pointing position defined by the
optical axis) and roll angle. (The X,Y are in units of 14.94733
arcsec (see below) and the ROLL steps are in units of 0.2076
degrees.) The event-rates file is then used to calculate the
live-time fraction at each of these positions. Finally the output
exposure map is constructed by moving the detector map to each
off-axis position, rotated to each roll angle, and adding the
detector map with the appropriate weighting factor to the exposure
map under construction.
A MORE DETAILED DESCRIPTION OF THE MAPS
The detector efficiency maps have been constructed in 13 channel
ranges for each of the two PSPCs. The naming scheme is such that
file
det_n_m_x.fits
contains the map for PSPC-x over the channel range n-m (out of a
full resolution of 256 PI channels).
The channel ranges are as follows:
Band Name PI range Energy
R1 8-19 0.11-0.284
R1L 11-19 0.11-0.284
R2 20-41 0.14-0.284
R3 42-51 0.20-0.83
R4 52-69 0.44-1.01
R5 70-90 0.56-1.21
R6 91-131 0.73-1.56
R7 132-201 1.05-2.04
These detector efficiency maps were created by using events
from the ROSAT all-sky survey in detector coordinates to
approximate a flat field. Point sources, particle contamination
and times of short-term noncosmic background enhancements were
excluded from the data set. Furthermore, an estimate of the
residual particle background contribution to the data was
subtracted.
Creating the maps from such a pseudo flat field has an
advantage over using the theoretical vignetting function in that it
accurately reflects all detector and telescope nonuniformities.
Specific examples of such nonuniformities are the shadowing by the
wires and ribs of the window support structure, electronic
``ghost'' images in the R1L (and R1) band, and variations in the
window thickness and therefore the detector quantum efficiency as a
function of position. The maps depend on the X-ray spectrum and
their creation for each pulse-height band reflects the average
spectrum of the soft X-ray diffuse background. This will create no
problems in the lowest pulse-height bands where the vignetting is
little changed over the energy range covered by the band. However,
for the highest pulse-height band, if the spectrum of an extended
object is much different from that of the SXRB, the vignetting
correction will lose accuracy.
The maps for the two PSPCs are not the same. Besides
detector-specific artifacts, there is a small shift in the position
of the window support structures and the windows have slightly
different thickness distributions. The main survey was done with
the first PSPC (~180 days), and there were ~11 days of survey with
the second PSPC to extend the sky coverage in exposure gaps of the
main survey. Because of this, data exist to create maps for both
detectors. However, the statistics of the second PSPC data are
obviously significantly worse than those of the main survey, and are
inadequate for determining the fine structure in the maps for bands
R3 through R7. For these bands, templates were created by shifting
the maps of the first PSPC to correctly align the shadows of the
window support wires and ribs with the second PSPC. The shifted
maps were then normalized to the maps of the second PSPC over
overlapping 5'x5' regions to give the correct telescope vignetting
and detector quantum efficiency. Unfortunately, the systematics of
the detector artifacts in the R1 and R2 bands are sufficiently
different between the two detectors to preclude using the same
scheme for these bands. So, despite the worse statistics, the maps
for the second PSPC for these bands were created using only the 11
days of survey data taken with this detector.
The pixel size of the maps is 14.947'' x 14.947''. The reason
for this somewhat obscure pixel size is that the PSPC detector
position digitization is 0.934208''and the detector coordinates
were binned by 16 for the maps. Note that this is not an integral
number of SASS event-position intervals (0.5''), or the same pixel
size as the SASS event images (15''x15''). The maps were
normalized to the on-axis value by fitting the radial distribution
of the inner 18' radius region of the PSPC to the theoretical
vignetting function (Molendi 1993). The *average* shadowing by the
window support wires is therefore not included in the exposure
correction; however, it is included in the window transmission for
modelling purposes. The spatial structure of the shadowing caused
by the window support wires and the window support ribs is included
in the exposure correction produced by the maps.
The effects of electronic ghost images are very obvious in the
regularly spaced bright spots and somewhat less-bright lines. The
PSPC is an imaging proportional counter that makes use of induced
charge on crossed cathode wires to obtain the position of accepted
events. The two-dimensional position determination is done using
the largest signals on the crossed cathodes, essentially
interpolating the event position between the two nearest cathode
wires in each direction. For very low pulse-height events, there
is the possibility that only one cathode in one or both directions
will have signals above the lower level discriminator of the analog
electronics chain. In this case, the position determination
degenerates to the center of the nearest cathode, yielding a line
(if only one axis has a single nonzero cathode value) or a point
(if both axes have only one nonzero cathode value each).
Also visible in the detector_maps is a slight bending of the
electronic ghost-image lines. This detector artifact is due to the
position correction algorithm. The algorithm corrects the event
position based on the assumption that the X-ray was absorbed in the
counter gas near the window. The bulging of the window support
structure by the pressure of the counter gas bends the electric
field lines in the electron drift region of the PSPC. This causes
a displacement of the event position, an effect which has been
calibrated and is included in the SASS event position-correction
procedure. Since the low pulse-height events which contribute to
the electronic ghost images have detected positions shifted to the
wire positions, this correction is not the appropriate one.
Electronic ghost images strongly affect only the R1 and R1L
band maps, although the R2 band map also shows some
irregularities. However, since the electronic ghost images are
pulse height and not energy dependent, the R1 map created from the
high-gain data is reasonably appropriate for correcting the R1L
band data collected in the low-gain state (where no survey data
exist to produce a flat field). This works because the R1L band at
low gain includes the same pulse heights at its low end as the R1
band at high gain. The weighting by the source spectrum is of
course slightly different, but this is a small effect in this
energy range. The R1 maps should be used for R1 band analysis for
data collected during high-gain operation. The R1 map should also
be used for R1L band analysis for data collected during low-gain
operation. We have created R1L maps for both detectors to be used
in R1L band analysis *only* for data collected during high-gain
operation. (Note that the R1L band nomenclature refers to the
11-19 channel band, which must be used for observations taken at low
gain. However, the R1L band can also be used for observations taken
at high gain. This is even preferable if an image derived from
high-gain data will be compared with an image derived from low-gain
data. To make sense of all this, remember that the PI channels are
adjusted to always correspond to the same energy, so the
corresponding pulse height will change with gain.)
The maps are described in more detail in Snowden et al 1994 (ApJ
424, 728), Snowden et al. (1992, ApJ, 393 819) and Plucinsky
et al. (1993, ApJ, 418, 519) and details of how to use these maps
in data analysis are provided in Snowden, et al 1994, ApJ, 424,
714.
WARNINGS ON USAGE
It should be noted that the o/p data array is written as REALS in
the Primary extension of the o/p FITS file. The image display &
manipulation task, saoimage (v1.06), is unable to correctly read
such datasets on DEC VMS & ultrix platforms.
The current version of this task will only operate on i/p
datasets in US Rev0 format or RDF format (ie cannot read German/UK
Rev0 datasets). Furthermore all i/p datasets must be in the same
format.
PARAMETERS
evrfil [character string]
The name of the FITS file containing the qualified event rate
data for the observation.For RDF format this is usually the
*_anc.fits file, and for US Rev0, it is the *.evr file.
attfil [character string]
The name of the FITS file containing the attitude data for the
observation. The special character % can be used to indicate
that the extension containing the attitude dataset is in the
same file as specified via the evrfil parameter, this is
usually the case for RDF data. For RDF format, this is usually
the *_anc.fits file, for US Rev0 it is the *.cas file.
gtifil [character string]
The name of the FITS file containing the Good Time Intervals
(GTIs) to be used. If the special strings 'NONE', 'none' or '
' are given, then the task will assume that all times given in
the EVR dataset should be used. For RDF format this is
usually the *_bas.fits file, and for US Rev0 it is the *.fits
file.
(evtfil=%) [character string]
The name of the FITS file containing the EVENTS data. This is
only used for US Rev 0 data. The sky coordinate values are
read from this file. If the special string "none" is entered
and the input datasets are in US Rev 0 format then the user
will be explicitly prompted for the pointing values.
qdetmap [boolean]
whether devignetted detector map to be used. If yes, it
chooses automatically which (hi or low) gain to be used.
dmapfil [character string]
The name of the detector map file.
outfil [character string]
The name of output file.
(clobber=no) [boolean]
Overwrite existing file ?
(chatter=9) [Integer]
Flag to indicate how chatty the task is at execution. A value
of 9 is the default, with lower/higher values producing
quieter/verbose output respectively.
EXAMPLE
% pcexpmap
Enter Event rate filename[] rp900176n00_anc.fits
Enter Attitude filename[] %
Enter GTI filename[] rp900176n00_bas.fits
Enter output filename[] rp900176n00_pcexpmap.fits
Want to use devignetted detector map?[yes] yes
** pcexpmap 2.3.2
... Number of unique detector positions 930
... Number of entries when Detector ON 24333
... Number of entries when Detector OFF 9 (ALL c/rate<10)
... Total ONTIME 24332.00000 s
... Total LIVETIME 23684.80664 s
... Average MV c/rate 81.82201 count/s
100% completed
** pcexpmap 2.3.2 completed successfully
BUGS
None known
SEE ALSO
Snowden et al. (1992, ApJ, 393 819)
Plucinsky et al. (1993, ApJ, 418, 519)
LOG OF SIGNIFICANT CHANGES
v2.1.0 (1996 Aug) Banashree Mitra Seifert
Added option to use devignetted map
v2.0.5 (1996 Jan)
Added parameters - ra_nom, dec_nom and evtfil
v2.0.0 (1994 Mar)
Added dynamic memory allocation, and renamed task from
PSPCEXPM
v1.0.0 (1993 Nov)
Beta-test version
PRIMARY AUTHOR
Rehana Yusaf
HEASARC
NASA/GFSC
http://heasarc.gsfc.nasa.gov/cgi-bin/ftoolshelp
(301) 286-6115